Hydrostatic piston machine construction

ABSTRACT

A disk-shaped control member of a hydrostatic axial or radial annular machine has a front face and a rear face. The front face is provided in each half thereof with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate the surface and the openings, and hydrostatic bearings in form of two pair of shallow circumferential recesses arranged with radial spacing at two diametrally opposite dead-center locations of the control member. The rear face is exposed to the atmosphere and has two pair of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter. These bores are each adapted to define with an inserted piston an annular gap. Channels connect the annular groove with the rear face and thus with atmospheric pressure. Passages connect the recesses with the respective annular gap.

Lachne r- [111 3,803,984 [451 Apr. 16,1974

[ 4] HYDROSTATI'C PISTON MACHINE CONSTRUCTION Y [75] Inventor:

[73] Assignee: Kloeckner-Werke Alrtiengesellschaft,

. Duisburg, Germany Hans Lachner, Heme, Germany [22] Filed: Nov. 20, 1972 [21] Appl. No.: 307,988

[30] Foreign Application Priority Data Nov. 20, 1971 Germany 2157704 52 Us. Cl. 91/487, 308/9 51 Int. Cl. ..F0lb-1/00,FO1b 13/04 [58] 1 Field of Search l37/625.21; 308/9; 91/487,

[56] References Cited Pastor ..91/'4"87 Primary ExaminerHenry T. Klinksiek Assistant Examiner-Robert J. Miller Attorney, Agent, or Firm-Michael S. Striker [5 7] ABSTRACT A disk-shaped control member of a hydrostatic axial or radial annular machine has a front face and a rear face. The front faceis provided in each half thereof with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate the surface and the openings, and hydrostatic bearings in form of two pair of shallow circumferential recesses arranged with radial spacing at two diametrally opposite dead-center locations of the control member. The rear face is exposed to the atmosphere and has two pair of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter. These bores are each adapted to define with an inserted piston an annular gap. (fliaifnels connect the annular groove with the rear face and thus with atmospheric pressure. Passages connect the recesses with the respective annular gap.

5 Claims, 4 Drawing Figures PATENTEUAPR 16 mm SHEET 2 (1F 3 I-IY'DROSTATIC PISTON MACHINE CONSTRUCTION I BACKGROUND OF THE INVENTION The present invention relates to piston machines, and more particularly to hydrostatic axial and radial piston machines. Still more particularly, the invention relates to a disk-shaped control member which is used in such piston machines.

Hydrostatic piston machines are well known in the art and are thus not believed to require detailed discussion. They use, as is also well known, control members which as a rule are relieved only hydraulically. Therefore, these control members do not have actual hydrostatic bearings with flow throttles. The reason why such hydrostatic bearings are not present in the prior-art constructions is that a simple slide bearing usually suffices to absorb the additional forces which result with reference to a minimum value, as a result of the respec' tive phase position of the outlet openings of the cylinderdrum of the machine with respect to the kidneyshaped control openings (or to the dead-center locations which are to be found intermediate the adjacent ends of these control openings), and due to the fact that the supporting capability of hydrostatic bearings is independent of the gap height between the cylinder drum and the juxtaposed surface of the control member. 1

However, the use of simple slide bearings leads to wear, unless special technical measures are taken to form a lubricant wedge. Even this approach, which is of course expensive, is not always satisfactory because at low rotational'speeds alubricant wedge cannot form and the bearings thus will not be protected against wear.

One approach suggested in the art to avoid these problems has proposed theuse of quasi-hydrostatic bearings. These were conceived to operate slightly throttled over-compensated bearings with intermittent supply. Experience has shown that such a construction does, indeed afford advantages over the previously mentioned type, because they stabilize the gap height between the juxtaposed surfaces of the cylinder drum and control member at least in certain defined RPM ranges. However, it has also been found that at low and at very high rotational speeds they do not operate reliably. At low speeds these constructions permit a strongly fluctuating and excessive leakage flow in the control gap; at very high speeds the supporting effect is insufficient because the feed-in time of fluid into the bearings is too short.

Finally, it has also been proposed to use hydrostatic bearings having fluid inflow throttles in form of nozzles or baffles or the like. None of these proposals have, however, been found acceptable in actual practice because the throttling cross-sections were very readily clogged with contaminants at the required small quantities of liquid flowing through them.

SUMMARY OF THE INVENTION Accordingly, it is a general object of the present invention to provide'a hydrostatic piston machine which overcomes the disadvantages outlined above with respect to the prior art. i

More particularly, it is an object of the invention to make it possible, in a piston machine of the type under discussion, to compensate the additional forces occurring in the control gap by means of hydrostatic bearings having inflow throttles.

In keeping with these objects, and otherswhich will become apparent hereafter, the invention provides in a reversible hydrostatic piston machine a combination comprising a disk-shaped control member having a front face and a rear face. The front face is provided in each half thereof with a kidney-shaped control open ing, an annular bearing support surface, an annular groove intermediate the surface and the openings, and hydrostatic bearings in form of two pair of shallow circumferential recesses arranged with radial spacing at two diametrally opposite locations of said control member.

- The rear face of the control member is exposed to atmospheric pressure and has two pair of mirror-symmetrically arranged pressure cylinder bores which are provided at the respective dead-center locations at opposite sides of a plane passing through the latter. Each of these bores is adapted to receive a piston and to define with the same an annular gap.

The annular groove is connected via channels or passages with the rear face and is thus at a atmospheric pressure. The recesses are connected via channels or passages with the respective annular gaps.

In order to assure that the recesses will actually operate as hydrostatic bearings, they have an inflow throttle associated with them which operates reliably irrespective of the speed of rotation of the machine. The annular gaps are so dimensioned that on the one hand they are able to supply the necessary substantial throttling effect for the relatively small quantity of liquid which is supplied to the hydrostatic bearings, but will on the other hand not be susceptible to clogging by contaminants. The latter characteristic is achieved in-that the pistons which are movable relative to the control member, will be moved to and fro one during each rotation. In addition, the amplitude of such movement changes constantly with the load acting on the drive shaft in dependence upon pressure and angle of displacement. Thus, each annular gap is constantly subjected to a cleaning action which prevents the accumulation of contaminants that could lead to clogging.

The hydrostatic bearings are arranged at the deadcenter locations; this avoids the disadvantages outlined with respect to the prior art, relative to the so-called additive surfaces which are located between the adjacent ends of the kidney-shaped openings The hydrostatic bearings are so dimensioned that a sufficient compensation of the action provided by the additive surfaces is assured without, however, perceptibly increasing the residual force which is to be absorbed by the axial bearing of the drive shaft of the piston machine.

According to the present invention the passages connecting the bearings with the cylinders are open to circumferential grooves formed in the inner circumferential surfaces bounding the respective cylinder bore. Such grooves therefore have the length of the annular throttle, that is the distance between the point of feeding of the pressure fluid (which takes place when the pistons are most'deeply inserted into the respective cylinder bores) and the circumferential groove. This permits a sensitive accommodation of the hydrostatic bearings via the liquid supplied by the annular throttles.

According to a further advantageous embodiment of the invention the effective surface area of each hydrostatic bearing may be substantially the same as the maximum additive surface area which is located between the adjacent ends of the kidney-shaped openings in circumferential direction of the control member, and between the annular groove and central bore for the drive shaft in radial direction of the control member.

The effective surface area of each hydrostatic bearing is so selected that, taking into account the pressure loss which occurs in the annular throttle between the piston and the wall of the cylinder at the minimum permissible gap height intermediate the cylinder drum and the juxtaposed front face of the control member, and further taking into account the leverage relationships of the acting forces relative to the respective reference point, it corresponds to the additive surface area. It is again point out that the term additive surface area here refers to that surface area which results of the phasedependent difference between the largest and the smallest effective support surface of a kidney-shaped opening as a result of the differential positions of the outlet bores of the cylinder drum.

Still a further concept of the invention proposes to connect one recess of each pair of recesses with the high-pressure control opening, and the other with the low-pressure control opening. The radially innermost recess of each pair is connected with the high-pressure control opening when control member is so positioned that the pair in question is located at one dead-center position of the control member, whereas the same recess is connected with the low-pressure control opening when the control member is turned so that the same pair of recesses is now located at the diametrally opposite other dead-center position. This permits a use of the piston machine in both opposite directions of rotation without having to forego the advantages achieved with the present invention.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its'construetion and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a plan view of the front face of a control member according to the invention;

FIG. 2 is a view similar to FIG. 1, but showing the rear face of the control member;

FIG. 3 is a fragmentary enlarged detail view on line Ill-Ill of FIG. 2; and

FIG. 4 is a fragmentary enlarged detail view on line IV-IV of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The drawing shows in FIGS. 1-4 a single exemplary embodiment, it being understood that details of the piston machine have been omitted because they are known per se. It will suffice to point out that the exemplary embodiment illustrates a control member such as is used in a hydraulic, axial piston machine. In the context of the known structural features of such a machine, it is the illustrated control member 1 that is novel.

Such piston machines have a drive shaft which is journalled at two locations in a housing. On the shaft there is mounted a cylinder drum which is provided with axial bores extending in parallelism with the drive shaft and being distributed about the same in form a ring of such bores. Each bore accommodates a piston which is reciprocable in dependence upon the extent to which a swash plate is inclined to the axis of the drive shaft. The cylinder drum is fixedly mounted on the drive shaft which passes through a bore 2 of the control member 1 from the front face of the latter (shown in FIG. 1) to and beyond the rear face thereof (shown in FIG. 2). Thus, the front face is juxtaposed with an axial endface of the cylinder drum. The control member is axially movable, but cannot rotate.

In accordance with the present invention the front face of the member I is provided in each half with a kidney-shaped control opening 3. These are concentric with the bore 2 and mirror-symmetrically located at opposite sides of a plane A-B which passes through the two diametrally opposite dead-center points or locations and which divides the member 1 into two control zones. The openings 3 are located on the same circle as the open ends of the cylinder bores provided in the -cylinder drum, in which the pistons are reciproeable.

The free ends of the openings 3 are identified with reference numeral 4; FIG. 1 shows that they are adjacent one another in the region of the upper dead-center point, and again in the region of the lower dead-center point. The circumferential spacing between the respective adjacent ends is so chosen that when the outlet openings of the cylinder bores in the cylinder drum rotate along and in registry with the'openings 3, a shortcircuiting between the opening acting as the highpressure opening and the one acting as the lowpressure opening is avoided. Advantageously, the openings 3 are fonned by material-removing methods, such as by turning; to improve the flow of working fluid through them, the middle regions of the openings 3 may be stepped intermediate the front and rear faces. The rear face (see FIG. 2) is provided with bores 6 communicating with the openings 3. Into the bores 6 are inserted bushings which serve to conduct working liquid between the control member 1 conduits or passages provided in the housing of the piston machine in known manner.

Needless to say, the openings 3 could be produced in other ways, for instance during casting, molding or the like of the member 1. What is important is that the described configuration of these openings advantageously influences the direction and quantity of liquid flow through them.

The front face is also provided with an annular groove 7; the latter is concentric with the bore 2 and the openings 3, being spaced radially from both. Bores or passages 8, which are offset through communicate with the groove 7 and extend to the rear face of the member 1 (see FIG. 2). Since the rear face.

is exposed to atmospheric pressure, the groove 7 is thus also at atmospheric pressure and separates the region of the front face having the openings 3 from the outer region having an annular bearing support surface 9.

The member 1 is further provided with hydrostatic bearings, in form of pairs of shallow recesses 10 and 11. These are also kidney-shaped and provided in pairs, with the two recesses of each pair being located at respective one of the dead-center locations and being spaced from one another in radial direction of the member l. The outer periphery of the surface 9 is bounded by a step 12 as best seen in FIGS. 3 and 4.

The rear face of the member 1 is shown in FIG. 2 In addition to the bores 6 it has cylinder bores 13 whose depth corresponds to about 4/5 the axial thickness of the member 1, as best seen in FIGS. 3 and 4. With reference to plane A-B these bores 13 are arranged in pairs (see FIG. 2), being located approximately in the regions of the free ends 4 of openings 3 but being radially outwardly offset from these free ends 4, as shown in FIG. 2. When the piston machine is assembled, pistons (not shown) are inserted into these cylinder bores 13; the purpose of these pistons is to press the member 1 against the juxtaposed endface of the cylinder drum when pressure fluid is admitted in requisite sense into the cylinder bores 13, or to provide-in cooperation with the bores 6- a force and moment compensation for the forces which are active in the region of the openings 3 and tend to shift the member 1 away from the cylinder drum.

It is evident that the pairs of cylinder bores are provided at therespective dead-center locations, and the drawing shows that they are each connected in crosswise relationship with that opening 3 which is located in the opposite half of the member 1, by the illustrated passages. This is shown in FIGS. 3 and 4, where it will be seen that from the rear face 5 there extends an inclined passage 14 to the inner endsection 15 of the respective cylinder bore 13; the outer end (located at endface S) is closed with a plug 15. Another passage, identified with numerals 17, communicates with the passage 14 and with the associated opening 3. Because of the crossover connection (which is plainly shown in FIG. 2) the connection of the other cylinder bore 13 with its associated opening 3 requires that, as shown in FIG. 4, a horizontal passage l8;passes radially through the portion 15 of the last-mentioned opening 3 from the periphery of the member 1 in radial direction; this passage 18 extends to the end region 4 of the opening 3 in question and also closed at its outer end with a plug 16. 1

Each half of the member 1 is further provided at rear face with two additional bores 19, as shown in FIG. 2; into these bores 19 are inserted helical expansion springs (not shown) whose purpose essentially is to press the member 1 against the cylinder drum when the piston machine is not in operation.

The recesses 10, 11 receive pressurefluid from the respective cylinder bores 13 and constitute hydrostatic bearings with respect of the endface of the cylinder drum (not shown) which is juxtaposed with them. These bearings serve to compensate for such irregularities as occur at the otherwise balanced member 1 in the dead-center locations, as a result of the phaseconditioned change from four to five pistons guided in the cylinder passages of an associated cylinder drum which may, for instance, have nine such passages.

. To assure that the recesses l0, 11 will effectively act as hydrostatic bearings, throttling means must be provided in the pressure-fluidsupply passages. In'the illustrated embodiments this is achieved in that each of the pistons to be associated with the member 1 is accommodated in a respective cylinder bore 13 thereof with a predetermined amount of play, such that an annular gap exists between the wall 20 of the respective cylinder bore 13 and the circumferential surface of the piston accommodated therein (not shown). This results in the formation of a throttling gap x which is shown in FIGS. 3 and 4, when the annular gap constitutes a length of the passages which conduct pressure fluid to the hydrostatic bearings 10, 11, i.e. to the recesses forming these bearings.

An annular groove 21 is formed in the circumferential wall bounding the respective cylinder bore 13; this groove 21 determined the length of the throttling gap x and bores 22 open into this groove 21. The bores 22 extend from the periphery of the member 1 but are closed again in the region of the periphery (see PK]. 2) after they have been formed; they communicate with the openings 3 via bores 23. Thus, the pressure fluid is throttled during its passage to the recesses 10, 11 to which it is supplied in the dead-center locations, assuring a direct dependence upon the respectively prevailing pressure of the pressure fluid. This is of course a significant throttling effect, which in turn makes the action of the hydrostatic bearings constituted by the recesses 10, 11 rather stiff, thereby making possible small vertical movements with large load force changes in the control gap.

I The drawing also shows that in this embodiment the arrangement and connection of the recesses 10, 11 with the openings 3 via the respective cylinder bores 13 is so selected that of the two recesses 10, 11 associated with each dead-center location, one is connected with the low-pressure opening 3 and the other is connected with the high-pressure opening 3. In particular, the radially innermost recess 11 of one pair is connected with the high-pressure opening 3, whereas the other similarly radially innermost recess 11 of the-other pair is connected with the low-pressure opening 3. The radially outer recesses 10, 11 are each connected with the openings 3 in the reverse relationship, i.e. with the lowpressure opening 3 and with the high-pressure opening 3, respectively.

The effective surface area of each of the recesses 10, 11 corresponds approximately to the additive surface area, taking into consideration the pressure loss which occurs in the throttle x between the wall of the respective cylinder bore 13 and the piston accommodated therein at the minimum permissible gap height between the cylinder drum and the member 1, and further taking into account the leverage relationship between the acting force and the respective reference point. The term additive surface area as employed herein should be understood to means that surface area which results from the phase-dependent difference between the greatest and the smallest effective supporting surface area of a respective kidney-shaped opeing 3, as a result of the differential position of the open ends of the cylinder bores in the cylinder drum with reference to'th openings 3.

It will be understood that each of the elements de scribed above, or two or more together, may also find a useful application in other types of other applications differing from the types described above.

While the invention has been illustrated and described as embodied in a hydrostatic piston machine, it is not intended to be limited to the details shown, since various modifications-and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can by applying current knowledge readily adpat it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention and, therefore such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims:

1. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite deadcenter locations of said control member; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.

2. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite deadcenter locations of said control-member; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each having an inner circumferential wall provided with a circumferential groove and each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps and each communicating with the respective circumferential groove.

3. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a control bore for a shaft, a front face each half of which is provided with a kidneu-shaped opening respective adjacent ends of which are spaced circumferentially from one another, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearing in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite deadcenter locations of said control member, the effective surface area of the respective hydrostatic bearings beingsubstantially equal to the surface area which is bounded in circumferential direction of said member by the respective adjacent ends of said openings and in radial direction of said member by said control bore and said annular groove; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.

4. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening of which one is a highpressure control opening and the other is a lowpressure control opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite deadcenter locations of said control member, one recess of each pair of recesses being connected to said highpressure control opening and the other recess of the same pair being connected to said low-pressure control opening; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.

5. A combination as defined in claim 4, wherein the radially innermost recess of the pair at one dead-center position of said member is connected with said highpressure control opening, and at the radially innermost recess of the pair at the diametrically opposite other dead-center position is connected with said lowpressure control opening. 

1. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite dead-center locations of said control member; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective deadcenter locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.
 2. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite dead-center locations of said control member; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each having an inner circumferential wall provided with a circumFerential groove and each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps and each communicating with the respective circumferential groove.
 3. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a control bore for a shaft, a front face each half of which is provided with a kidneu-shaped opening respective adjacent ends of which are spaced circumferentially from one another, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearing in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite dead-center locations of said control member, the effective surface area of the respective hydrostatic bearings being substantially equal to the surface area which is bounded in circumferential direction of said member by the respective adjacent ends of said openings and in radial direction of said member by said control bore and said annular groove; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.
 4. In a reversible hydrostatic piston machine, a combination comprising a disk-shaped control member having a front face each half of which is provided with a kidney-shaped opening of which one is a high-pressure control opening and the other is a low-pressure control opening, an annular bearing support surface, an annular groove intermediate said surface and openings, and hydrostatic bearings in form of two pairs of shallow circumferential recesses arranged with radial spacing at two diametrically opposite dead-center locations of said control member, one recess of each pair of recesses being connected to said high-pressure control opening and the other recess of the same pair being connected to said low-pressure control opening; a rear face exposed to atmospheric pressure and having two pairs of mirror-symmetrically arranged pressure cylinder bores provided at the respective dead-center locations at opposite sides of a plane passing through the latter, said cylinder bores each being adapted to define with a respectively inserted piston an annular gap; first passages connecting said annular groove with said rear face; and second passages connecting said recesses with respective annular gaps.
 5. A combination as defined in claim 4, wherein the radially innermost recess of the pair at one dead-center position of said member is connected with said high-pressure control opening, and at the radially innermost recess of the pair at the diametrically opposite other dead-center position is connected with said low-pressure control opening. 